Method of making a controlled-diffusion stippled reflector by sag molding

ABSTRACT

For a lamp unit, a sagged glass reflector having a stippled, concave surface for providing controlled diffusion of light. The method of making the reflector comprises: providing a preheated concave mold having a uniform pattern of peened indentations in its surface and a plurality of vacuum drawing holes; placing a flat blank of glass across the opening of the mold; heating the glass to a plastic state; drawing a partial vacuum in the mold to force the plasticized glass to sag against the peened surface of the mold; cooling the glass to a rigid state; and applying a coating of reflective material on the concave surface of the sagmolded glass.

Levin 1 June 24, 1975 METHOD OF MAKING A CONTROLLED-DIFFUSION STIPPLED REFLECTOR BY SAG MOLDING Inventor: Robert E. Levin, South Hamilton,

Mass.

Assignee: GTE Sylvania Incorporated, Salem,

Mass.

Filed: Aug. 23, 1973 Appl. No.: 390,871

Related US. Application Data Division of Ser, No. 319,321, Jan. 3, 1973, Pat. No. 3,825,742,

US. Cl. 65/107; 65/273; 65/285; 65/60 Int. Cl C03b 23/02 Field of Search 65/106, 107, 273, 275, 65/285, 60

References Cited UNITED STATES PATENTS 8/1917 Hough, Jr. 65 /l06 1,884,665 10/1932 Greiner 65/60 Primary ExaminerArthur D. Kellogg Attorney, Agent, or FirmEdward .1. Coleman [57] ABSTRACT For a lamp unit, a sagged glass reflector having a stippled, concave surface for providing controlled diffusion of light. The method of making the reflector comprises: providing a preheated concave mold having a uniform pattern of peened indentations in its surface and a plurality of vacuum drawing holes; placing a flat blank of glass across the opening of the mold; heating the glass to a plastic state; drawing a partial vacuum in the mold to force the plasticized glass to sag against the peened surface of the mold; cooling the glass to a rigid state; and applying a coating of reflective material on the concave surface of the sag-molded glass.

9 Claims, 6 Drawing Figures METHOD OF MAKING A CONTROLLED-DIFFUSION STIPPLEI) REFLECTOR BY SAG MOLDING This is a division of application Ser. No. 319,321. filed Jan. 3, 1973, now U.S. Pat. No. 3,825,742 issued July 23, 1974.

BACKGROUND OF THE INVENTION This invention relates to lighting devices in which a reflector is used with a lamp and, more particularly, to an improved reflector for providing controlled diffusion of light and a method of making such a reflector. Controlled diffusion is defined as diffusion that evens out the beam pattern and eliminates filament images without destroying the basic spacial luminous flux distribution from the lamp. A reflector having such light diffusion characteristics is useful in a variety of applications, e.g., for floodlighting, and in the condensing optics for projection devices.

Reflectors have been formed in a multitude of manners, such as spinning, electro-forming, hydro-forming, explosion forming, stamping, etc., if the material was properly workable, such as aluminum. Glass reflectors have been formed by blowing or gravity sagging into a mold or by pressing between a two-part mold. Techniques such as grinding and polishing are not applicable to low cost, large volume production of nonspherical elements.

For precise control of light, the reflector surface must be specular in nature. However, it is often necessary to smooth out irregularities in the light distribution by causing the spread of light about the specular direction from each point on the reflective surface within a welLdefined cone. The common diffusion techniques using processes such as etching, sand-blasting, etc., cause an uncontrolled diffusion over large angles, typically as much as 211- steradiansv The control desired can be achieved by forming small (with respect to the total reflector) zones or greater or less curvature than the basic reflector at a multitude of adjacent points over the reflective surface. In the past this has been done by forming the negative of the small elements in a forming tool that mates with the reflective surface. Thus, this technique has been applied to the aforementioned metal forming techniques and to pressed glass components.

A controlled-diffusion glass reflector is particularly useful when the heat must be reduced in the light beam. A glass reflector with a cold dichroic mirror coating will reflect the visible light while passing a major portion of the infrared power through the reflector. This separation technique is not possible with metal reflectors, where the most that might be provided is for the infrared power to be absorbed by a dichroic coated metal mirror, raising local temperatures. However, this process generally is not considered functional at the present time.

As a specific example of an application of light diffusing reflectors and the problems associated therewith, consider a reflective condensing system for a 35mm slide projector in which the reflector is integral to the lamp. Such a system includes; an objective lens, a film gate aperture, a relay lens, and a lamp assembly with a reflector. For a lamp in the 250 watt range with convention projection optics of a 4 inch, f/3.5 objective and a relay lens of about diopters paraxial, and with the normal space limitations, a reflector-condenser system requires controlled spreading of light within limited solid angles to prevent localized nonuniformities on the screen. Previously, this was accomplished with pressed reflectors which were heavy, were rim- 5 mounted as opposed to base-and-socket mounted, and

relative expensive. The engraving of the male tool exhibited wear that caused progressive changes in successive lamps and was difficult to replicate due to its fine structure.

SUMMARY OF THE INVENTION In view of the above-discussed disadvantages of the prior art, it is an object of the present invention to provide an improved controlled diffusion lighting device.

Another object ofthe invention is to provide a reflec tor of glass or a similar material having an improved stippled surface over a substantial portion thereof for providing controlled diffusion of light.

A further object of the invention is to provide improved means for making such a reflector.

In accordance with the invention, I have discovered that by sagging a plasticizable material, such as glass, into a concave mold having a stippled surface, and thereafter coating the concave surface of the sagged piece with a reflective material, a reflector can be provided which has controlled local zones of increased or decreased curvature, as compared to the general reflector curve at the same points, for providing a con trolled diffusion of reflected light. The sagging process comprises forcing a material, such as glass, in the plastic state against the stippled surface of a mold by gas pressure and/or gravity such that the mold is only in contact with the surface opposite the final reflective surface. In this manner the final reflective surface is uncontaminated by physical contact with tools. The resulting stippled reflecting surface forms a spread of light from each part of the reflector which is confined within well defined spatial regions.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a lamp unit having a sagged reflector according to the invention;

FIG, 2 is a front view of the lamp unit of FIG. 1;

FIG. 3 is a simplified cross-section of a mold used in making the reflector of FIGS. 1 and 2;

FIG. 4 is a partial section view of the molded glass used in the reflector of FIGS. 1 and 2;

FIG. 5 is a magnified view of a region of the reflector cross-section of FIG. 4; and

FIG. 6 is a diagram illustrating the optical effect of a peen on a reflecting surface.

DESCRIPTION OF PREFERRED EMBODIMENT A reflector according to the invention is formed by sagging a material, typically glass, heated beyond the softening point into a mold using gravity and/or a pressure differential between the surrounds and the region contained between the glass and the mold. The mold differs from the final reflective surface by the thickness of the glass with allowances for the variable glass thickness due to glass thickness due to glass flow during the forming. Small, local variations in curvature, hereinafter referred to as stippling, are provided over the mold surface. Generally, this stippling takes the form of concave spherical indentations on the mold surface, but other shapes such as spheroidal, ellipsodial, flat of nonconic may be used; also, convex elements may be used.

The local zones, comprising the stippling, or peens, are of specified size and shape as well as being arranged in some specified orderly fashion such that the amount of localized spread can be controlled as well as permitting convenient replication of molds. The peens are conve niently specified in terms of the forming tool (e.g., for a spherical peen, by the radius of the sphere) and the average spot diameter.

The formulation of the peens may vary over the reflecting surface depending on the degree of spread required for the light rays in a particular direction. The reflecting surface, which is ultimately coated with a reflective material such as aluminum, silver, an interference filter, etc. is free to too] marks and other irregularities commonly developed during intimate contact with forming tools.

A projection lamp unit employing a reflector according to the invention is shown in FIGS. 1 and 2. An ellipsoidal reflector is coated with a dichroic cold mirror on the interior concave surface 12 which carries the stippling, or peens, providing the optical control. Peens of greater curvature on the outer convex surface 14 are produced by direct transfer from a female mold. The lamp base 16 is used to position and support the lamp 18 in the reflector. Lamp 18 is typically a tungsten halogen lamp with an incandescent filament and is introduced through a hole 20 at the apex of the reflector. The lamp may be supported within or in immediate proximity to the volume defined by the reflector l2 and the aperture plane thereof.

A typical arrangement for the sagging process is illustrated in FIG. 3, which shows a simplified cross-section through the center of a mold 22 for sag-forming a stippled reflector according to the invention. The concave surface 24 of the mold, which mates with the outer reflector surface during sagging, is peened to provide the desired stippling configuration, represented by the spherical indentations 26. Preferably, this stippling is disposed in a non-random pattern; hence, the mold surface 24 contains a substantially uniform pattern of spherical indentations 26. To enable a vacuum to be drawn between the mold and the sagged glass, the stippled surface of the mold contains a plurality of small holes, in the order of 0.020 inch diameter, as illustrated by a typical hole 28. Relief 30 in the central portion of the mold produces additional thinning of the glass in that region where the central hole 20 (FIG. 2) is to be formed in the reflector. The shape of this relief is not critical. Minor variations in the upper edge of the mold are dictated by the processes used to cut and finish the glass after molding.

A preferred method of making a stippled reflector according to the invention will now be described. First, the above described mold is preheated to between I l00 and 1200C. Next, a flat glass blank 32 is placed across the opening of the preheated mold 22 with suff cient overlap to provide a seal thereabout. Typically the glass is about 0.070 inch thick and comprises ordinary soda lime glass having a mean coefficient of thermold without tools, whereby the glass is formed to have a untooled concave surface with stippling thereon. The creation of a vacuum assumes an intimate contact between the glass and the mold wiithout the voids that may occur during a pure gravity sag. Typically, in a production process, the above steps are accomplished by carrying several molds on a conveyor belt through a furnace having a controlled temperature in the range from about l l00 to I200C. The sag-molded glass is then permitted to cool to a rigid state.

FIG. 4 is a partial section view illustrating the thickness variation of the molded glass 34. The thin central portion 36 formed by relief 30 is subsequently cut off at about 38 to provide hole 20. A magnified view of one region 40 of the glass cross-section is shown schematically in FIG. 5. The outer, or convex. surface 42 is formed by direct contact with the mold, while the optically weaker stippling elements 26' are formed on the interior surface 44 by the plastic flow of the glass. A thin deposited film 46 of reflective material such as aluminum, is coated on the inner surface 44 of the molded glass as the light reflecting component. The coating 46 may be a cold dichroic mirror coating for reflecting visible light from the incandescent filament of lamp 18 but passing infrared radiation back through the reflector.

A basic reflector shape (FIGS. 1 and 2) suitable for the 35 mm projector optics described hereinbefore is a prolate ellipsoid of major diameter 1.439 inch and minor diameter 0.922 inch. A female mold 22 made for forming this reflector allowed for an apex glass thickness of 0.024 inch and a positive rate of change of thickness of 0.038 inch per inch as a function of axially measured distance from the apex. Thus, the mold was cut back from the desired ellipsoid by this amount. The specific values used are based on the type and original thickness of the flat glass blank used plus the temperature distribution and processing times and pressures of the sagging operation. Thirty-two vacuum drawing holes 28 were arbitrarily spaced about the mold surface to provide an equal vacuum. The degree of peening on the reflective surface is generally subjective since no specifications commonly exist for local uniformity in terms of the generally objectionable banding, spotting, and other striations that are controlled by peening. The uniformity between various parts of the projection screen as defined by corner-to-center ratio is not the principal criterion for peening. The necessary amount of controlled diffusion provided by the peening was arrived at by a consensus of experts on the acceptability of projection systems. This is the normal method of determining acceptability. Allowing for the decrease in optical strength of a peen formed on the back surface and transferred to the front reflecting surface by the flow of glass, the required rear surface peens were produced by forming nonrandom peens on the mold of 0.625 inch spherical diameter and of mean circular diameter of0.() inch. This provides a non-random, substantially uniform pattern ofconcave spherical indentations 26' on the concave surface of the sagged glass.

The functioning of the stippling, or peens, is illustrated in FIG. 6. Consider a pencil of light rays 48, incident on a smooth reflective surface 50. The reflected pencil 52 will be at the specular angle with virtually no change in divergence. If the source is nonuniform in luminance. such as coiled incandescent filament, these non-uniformities may image" in a crude sense produc ing the undesirable uneven illumination field. If a peen 54 is located at the point of reflection, then the reflected pencil will have significant divergence, say through the solid angle 56. This tends to destroy any partial imaging by the system. Nevertheless, the incident pencil is still directed predominately in the specular angle direction. If any form of general diffusion were used, the reflected light would be spread through a large angle approaching 21-r steradians thereby lessening or eliminating the directional control of the reflector. Accordingly, specularly reflecting stippling is disposed over the sagged glass reflecting surface to provide improved directional control for application such as floodlights and projection lamps.

In summary, the present invention relates to specular reflector of glass or similar materials formed in the plastic state by forcing against a mold by gravity and/or gas pressure, the surface in contact with the mold being opposite to the reflective surface, and the reflective surface being coated with a reflective material such as aluminum. Further, a degree of controlled spread of the light is provided by forming gross surface irregularities while maintaining the specular nature of the surface at all local points. The reflector is generally used for the control of light either in conjunction with, or as an integral part of, a lamp or luminous source. There are specific advantages to the sagging techniques that make it advantageous in many situations. The final reflective surface is untouched by the forming tool; thus, if the original surface is of high quality, it is not degraded by contact with forming tools. Economic advantages exist for some sizes and shapes over other methods of production. Further, the use of saggeo' glass with stippling to provide a controlled diffusion reflector is particularly advantageous with respect to heat control and heat resistance. More specifically, glass permits the use of a dichroic coating for reducing heat in the light beam without causing the localized heating of a metal reflector, and the sagging process permits the use of much thinner glass than is typically required for pressed glass reflectors. For example, the maximum thickness of the sagged glass reflector is normally much less than 0.! inch, whereas a pressed glass reflector is typically much thicker than 0.1 inch, for example, 0. l 8 inch. As thinner glass has a much higher thermal shock resistance, the sagged reflector may be formed of the less expensive, higher expansion glasses, such as ordinary soda lime glass. Due to the lower thermal shock resistance, the thicker pressed glass reflectors typically employ the more expensive, low expansion, borosilicate glasses to lower the stress levels due to heat.

Although the invention has been described with respect to specific embodiments, it will be appreciated that modifications and changes may be made by those skilled in the art without departing from the true spirit and scope of the invention.

I claim:

I. A method of making a stippled reflector for providing controlled diffusion of light, said method comprising:

providing a preheated concave mold having a stippled surface, said stippling on said mold surface comprising a pattern of localized zones ofa change in the radius of curvature of the surface of said mold with respect to the large scale concave curvature thereof;

placing a flat blank of rigid material across the opening of said preheated mold, said material being of a type which may be heated to a plastic state for forming; heating said material to a plastic state; forcing said plasticized material to sag against the stippled surface of said mold without tools, whereby said material is formed to have an untooled concave surface with stippling thereon;

permitting said sag-molded material to cool to a rigid state;

and applying a coating of reflective material on the stippled concave surface of said sag-molded material.

2. The method of claim 1 wherein said flat blank of rigid material is a sheet of glass.

3. The method of claim 2 wherein said glass is transformed to a plastic state by heating it to form about to l200C.

4. The method of claim 1 wherein the stippled surface of said mold has a plurality of holes through which a vacuum may be drawn, and wherein said step of forcing said plasticized material to sag against the stippled surface of said mold comprises drawing a partial vacuum in the space between said blank of material and said stippled mold surface.

'5. The method of claim 1 wherein the stippling on said mold surface is disposed in a non-random pattern.

6. The method of claim 5 wherein said stippling on said mold surface comprises a substantially uniform pattern of concave indentations in the surface of said mold.

7. The method of claim 6 wherein said flat blank of rigid material is a sheet of glass having a maximum thickness of less than about 0.1 inch.

8. The method of claim 7 wherein said step of heating said glass to a plastic state comprises placing said mold with the glass thereon in a furnace having a temperature of from about I IOO to 1200C.

9. The method of claim 8 wherein the stippled surface of said mold has a plurality of holes through which a vacuum may be drawn, and wherein said step of forcing said plasticized glass to sag against the stippled surface of said mold comprises allowing said plasticized sheet of glass to gravity sag and simultaneously drawing a vacuum in the space between said sheet of glass and the stippled surface of said mold. 

1. A method of making a stippled reflector for providing controlled diffusion of light, said method comprising: providing a preheated concave mold having a stippled surface, said stippling on said mold surface comprising a pattern of localized zones of a change in the radius of curvature of the surface of said mold with respect to the large scale concave curvature thereof; placing a flat blank of rigid material across the opening of said preheated mold, said material being of a type which may be heated to a plastic state for forming; heating said material to a plastic state; forcing said plasticized material to sag against the stippled surface of said mold without tools, whereby said material is formed to have an untooled concave surface with stippling thereon; permitting said sag-molded material to cool to a rigid state; and applying a coating of reflective material on the stippled concave surface of said sag-molded material.
 2. The method of claim 1 wherein said flat blank of rigid material is a sheet of glass.
 3. The method of claim 2 wherein said glass is transformed to a plastic state by heating it to form about 1100* to 1200*C.
 4. The method of claim 1 wherein the stippled surface of said mold has a plurality of holes through which a vacuum may be drawn, and wherein said step of forcing said plasticized material to sag against the stippled surface of said mold comprises drawing a partial vacuum in the space between said blank of material and said stippled mold surface.
 5. The method of claim 1 wherein the stippling on said mold surface is disposed in a non-random pattern.
 6. The method of claim 5 wherein said stippling on said mold surface comprises a substantially uniform pattern of concave indentations in the surface of said mold.
 7. The method of claim 6 whereiN said flat blank of rigid material is a sheet of glass having a maximum thickness of less than about 0.1 inch.
 8. The method of claim 7 wherein said step of heating said glass to a plastic state comprises placing said mold with the glass thereon in a furnace having a temperature of from about 1100* to 1200*C.
 9. The method of claim 8 wherein the stippled surface of said mold has a plurality of holes through which a vacuum may be drawn, and wherein said step of forcing said plasticized glass to sag against the stippled surface of said mold comprises allowing said plasticized sheet of glass to gravity sag and simultaneously drawing a vacuum in the space between said sheet of glass and the stippled surface of said mold. 